nx-match: New helpers.

nx-match: New helpers.

This patch creates two new helper functions, nxm_reg_load() and
nxm_dst_check().  The new nxm_dst_check() function may be used to
check the validity of destination fields used by actions.  The new
nxm_reg_load() function may be used by actions which need to write
to NXM fields.

This patch also allows multipath and autopath to write their result
to non-register NXM fields.